When we think of light, we think of sunshine or the colors of the rainbow. But colors, like sounds, are limited by the range of our senses. Just as there are sounds that we cannot hear because the pitch is too high or low for our ears to detect, so there are colors whose color pitch is too high or low for our eyes to detect. We think of light as the colors of the rainbow, which really is just a tiny part of a much broader spectrum that our eyes cannot see. We call this broader spectrum the Electromagnetic Spectrum. When sound is too high-pitched to be heard, it is called ultrasonic. The Electromagnetic Spectrum outside the visible is subdivided into several parts that also have special names: radio waves, microwaves, infrared, ultraviolet, x-rays and gamma rays. Despite the variety of names, they are all forms of light. How do the parts of the Electromagnetic Spectrum differ? In order to get a sense of the properties of light, let's examine the wave diagram in figure 1. To use this image, rotate the book so that its spine is horizontal. Move your finger along the wave from left to right. The distance between one crest of the wave and the next is called the wavelength. On the left, the wavelength is long, and as you move to the right, the wavelength shrinks and the crests become more frequent. These are two important features of waves. As a wavelength shrinks, the number of wave crests, their frequency, goes up. There is one other very important property of a wave. The higher the frequency of the crests, or the shorter the wavelength, the more energy the wave is carrying. Imagine jumping up and down trying to avoid the wave crests as they move past you. You'd need a lot more energy to avoid the crests because they're passing by much more frequently. The different names in the Electromagnetic Spectrum represent different wavelengths or energies of light. As you move your finger along the wave represented in Figure 1, the longest wavelength, lowest frequency and lowest energy waves are called radio waves. Next up are microwaves, then infrared light, visible light, ultraviolet light, x-rays and finally gamma rays, which have the shortest wavelength, highest frequency and most energy of all the forms of light. The wave you're touching is not to scale. Radio waves can be meters long, and gamma rays have wavelengths smaller than an atom. But they are all made of the same thing: light. And they all travel at the same speed: the speed of light. The speed of light is fast: 300,000 kilometers (about 186,000 miles) per second through space. Yet it still takes light hundreds, thousands or even millions of years to travel from a star or galaxy to us. The Universe is huge. Every object in the Universe emits light. The type of light it emits depends on its temperature and what it's made of, as well as other properties such as magnetic fields. Let's move through the Electromagnetic Spectrum and see how each type of light can be generated, and then we'll go off and explore the Universe.

Radio Waves:

Radio waves are the lowest-energy form of light. They are produced by electrons spiraling around magnetic fields. Magnetic fields are generated by stars, including our sun, and many weird celestial objects like black holes and neutron stars.

Microwaves:

Microwaves are high-energy radio waves Microwaves come from very cold gas with temperatures as low as -200 degrees Celsius. (That's -390 degrees Fahrenheit.) Cold giant molecular clouds are the birthplace of new stars.

Infrared:

We think of infrared as heat. Infrared light is emitted by objects that are warmer than the cold expanse of space but not hot enough to glow with visible light. Giant molecular clouds of dust and gas, despite having temperatures of -100 degrees Celsius, (that's -150 degrees Fahrenheit) still radiate a lot of infrared light. Some red stars, with surface temperatures of 1000 degrees Celsius, (that's 1800 degrees Fahrenheit) are very dim in visible light but shine brightly in infrared light. Most things in the Universe, including people, are sources of infrared light.

Visible Light:

Visible light is a very narrow portion of the Electromagnetic Spectrum. The sun, with a surface temperature of 6000 degrees Celsius, (that's 11,000 degrees Fahrenheit) gives out most of its light in the visible region, which is probably the reason our eyes evolved the way they did.

Ultraviolet:

Hot objects, such as the sun, radiate ultraviolet light. Fortunately our atmosphere blocks most of the sun's ultraviolet, otherwise sunburns would be much worse. Some stars are so hot, with surface temperatures up to 40,000 degrees Celsius, (72,000 degrees Fahrenheit) that most of their light is ultraviolet.

X-ray:

X-rays are very high-energy light waves, generated in extreme environments when temperatures are at millions of degrees. Colliding galaxies, exploding stars, and gas falling into black holes are some of the sources of x-rays.

Gamma Rays:

Light with the most energy is called gamma radiation. Exploding stars called supernovae generate a lot of gamma radiation, as do the nuclei of unstable, radioactive atoms that are created in supernova explosions.

All these types of light, except visible light, are invisible to us. The next chapter describes incredible telescopes that are designed to detect the invisible Universe.